1. Field of the Invention
This invention is related in general to the functionalization of the surface of materials for the purpose of improving their properties for particular applications. In particular, it pertains to a combined plasma-treatment/vapor-deposition process for functionalizing paper, membranes, and other woven and non-woven porous materials.
2. Description of the Related Art
The term “functionalization” and related terminology are used in the art and herein to refer to the process of treating a material to alter its surface properties to meet specific requirements for a particular application. For example, the surface energy of a material may be treated to render it particularly hydrophobic or hydrophilic as may be desirable for a given use. Thus, surface functionalization has become common practice in the manufacture of many materials because it adds value to the end product. In order to achieve such different ultimate results, functionalization may be carried out in a variety of ways ranging from wet chemistry to various forms of vapor deposition, vacuum metallization and sputtering.
Textiles, non-woven products and paper substrates are fiber-based porous materials with inherent properties derived from the nature of the fibers. Synthetic and natural fibers (for example, polypropylene, nylon, polyethylene, polyester, cellulosic fibers, wool, silk, and other polymers and blends) can be shaped into different products with a great range of mechanical and physical properties. In addition, the porosity of these materials usually serves a necessary function, such as gas and/or liquid permeation, particulate filtration, liquid absorption, etc. Therefore, any subsequent treatment designed to further modify the chemical properties of the fibers by appropriately functionalizing them must be carried out, to the extent possible, without affecting the porosity of the material. This has heretofore been virtually impossible when such functionalization results from the deposition of polymers.
A variety of wet chemical processes have been used traditionally to treat with polymers and functionalize fibers that are otherwise inert or have limited surface functionality. These processes involve the immersion of the fibrous material in liquids or fluid foams designed to coat individual fibers and impart specific functionalities while retaining the material's porosity and ability to breathe. In spite of the many claims made in commercial products, though, it is clear that such wet-chemistry processes at best materially reduce the porosity of the substrate or, in the worst cases, essentially plug the interstices between fibers. Therefore, the functionalization of porous materials by wet-chemistry polymer deposition has produced the desired results in terms of surface functionality, but with the attendant serious deterioration of the mechanical characteristics of the underlying porous substrate.
Thus, prior-art processes for functionalizing porous materials by coating the fibers with a polymer film have produced unsatisfactory results because of loss of porosity. In addition, these solvent-based and water-based processes for woven and non-woven fabrics, paper and other porous materials (like open- and closed-cell plastic foams) have been increasingly facing environmental challenges and constraints that result in higher end-product costs. In some cases, producers have actually withdrawn from the market coatings that present potential health hazards, such as the fluoro and chloro monomer materials used to functionalize products for hydrophobic/oleophobic and biocide properties, respectively.
Therefore, there is a pressing need for new coating technologies that are suitable for porous materials, are safe to implement, do not utilize solvents, and do not effect the mechanical and functional properties of the porous substrate. While polymers applied by vacuum deposition have been used successfully in the art to impart particular functional properties to non-porous, non-permeable substrates, no attempt was historically made to so functionalize porous materials because the vacuum deposition process was believed to be likely to exacerbate the pore plugging problem.
For example, the vacuum deposition of a polymer coating by flash evaporation of a monomer and its subsequent polymerization by radiation curing in a vacuum chamber has been used widely with a variety of monomers, such as free-radical polymerizable acrylates, cationic polymerizable epoxies and vinyl monomers, to control the surface energy of the resulting products and introduce desirable characteristics. Without limitation, these include hydrophobicity, oleophobicity, hydrophilicity, oleophilicity, fire resistance, biocidicity, color, anti-stain, antistatic, and sensor properties. In all cases, the substrate is exposed to a dense fog of vaporized monomer under conditions that cause its immediate condensation and curing on the substrate's surface. Therefore, it stood to reason to believe that these conditions would favor the accumulation of monomer droplets in the pores of a porous substrate and cause it to become impermeable. This invention is based on the surprising discovery that, when appropriately controlled, vacuum deposition can be use successfully to functionalize porous materials while retaining their permeability properties.